1. Field of the Invention
The present invention relates to radar signal processing systems. More specifically, the present invention relates to phase error correction in range migration algorithm (RMA) for synthetic aperture radar (SAR) systems.
2. Description of the Related Art
In imaging applications such as ground mapping, a radar system is used to generate a two-dimensional image of a portion of a ground surface in the range and azimuth (cross-range) directions. A large antenna aperture is required in conventional imaging radar systems in order to achieve a narrow beamwidth and, consequently, fine azimuth resolution. Synthetic aperture radar (SAR) systems have been developed as an alternative means for improving azimuth resolution by synthesizing pulse-to-pulse return signals collected by a moving platform with a small antenna. The signal synthesis from many successive locations of the moving platform accomplishes what would otherwise require a larger antenna aperture.
Polar format algorithm (PFA) has been widely used for high-resolution SAR systems. However, PFA has the shortcomings of limited depth of focus and geometric distortion that increases with the map size. Range migration algorithm (RMA) is one of the most attractive and advanced SAR processing techniques to avoid the problems with the PFA. A difficulty with the RMA, however, is performing efficient phase correction.
Normal SAR data collection requires phase coherence, not only within each pulse for range resolution, but also from pulse to pulse over the collection time needed for azimuth resolution. The platform position affects the pulse-to-pulse phase coherence over the synthetic aperture. Phase error introduced by the inaccuracy of navigation data or undesirable platform motion causes smearing or duplication of the target image. Since motion compensation at the early processing stage based on the navigation data is not sufficient for producing a focused image, it is a common practice to employ data driven autofocus algorithms in high resolution SAR systems in order to maintain phase coherence and achieve good image quality.
Considering the computational efficiency and simplicity, it is desirable to implement the autofocus function after range compression during batch processing as is usually done for the case of PFA. However, for the case of RMA, it is difficult to implement the autofocus function during batch processing because the signal support areas from different targets are not aligned. For this reason, the autofocus function had to be implemented before batch processing starts, at the cost of increased complexity and processing time.
Because of the difficulty in implementing the autofocus function during batch processing, current RMA systems perform the autofocus function during the pulse-to-pulse processing phase using a separate polar format processing algorithm. This approach, however, has the disadvantage of implementation complexity and tighter processing timeline requirements. Furthermore, the increased timeline requirement makes it more difficult, if not impossible, to implement more advanced autofocus techniques.
Hence, a need exists in the art for an improved system or method for efficient phase error correction in range migration algorithm.
The need in the art is addressed by the present invention, a system and method for efficient phase error correction in RMA implemented by making proper shifts for each position dependent phase history so that phase correction can readily be performed using the aligned phase history data during batch processing. In its simplest form, the invention is comprised of two main parts. First, alignment of the phase error profile is achieved by proper phase adjustment in the spatial (or image) domain using a quadratic phase function. Then, the common phase error can be corrected using autofocus algorithms.
Two alternative embodiments of the invention are described. The first embodiment adds padded zeros to the range compressed data in order to avoid the wrap around effect introduced by the FFT (Fast Fourier Transform). This embodiment requires a third step: the target dependent signal support needs to be shifted back to the initial position after phase correction. The second embodiment uses the range compressed data without padded zeros. Instead, an aperture of greater length needs to be generated by the Stolt interpolation. In this embodiment, the third step of shifting the signal support back can be eliminated.